Flexible-Link Velocity-Bounding Proxy Based Sliding Mode Control

Abstract

This paper proposes a control strategy for flexible link manipulators preserving high tracking accuracy in free motion, while ensuring smooth and safe recovery in scenarios involving physical interaction or large positional errors, based on VB-PSMC. The scheme is extended to compensate for the manipulator's flexural dynamics, resulting in a nested control scheme where damping of the induced oscillations is achieved by a model-free proportional strain feedback while gravity induced deflections are counteracted by a feed-forward term based on a quasi-static Euler-Bernoulli beam model. A convergence study on the modified sliding manifold and a stability analysis of the closed-loop system are provided. The performance of the controller was evaluated experimentally and compared against other control strategies such as PSMC and torque limited PD control. The results demonstrate the controller's accurate end effector tracking in free motion, while achieving compliant behavior during contact, by efficiently handling the link’s inherent flexibility leading up to a 32% reduction in interaction force. In addition, studying the FL-VB-PSMC response after releasing contact demonstrated the overdamped and vibration-free recovery even for large position errors.